In the last years different projects were launched with the aim to develop emission free gas turbine based processes using semi-closed cycles with CO2/H2O mixtures as working fluid. Methods of operating such power plants are known for example from EP-A1-0 939 199 and EP-A1-0 953 748. In these processes the fuel, usually natural gas, reacts with technically pure oxygen generated either in an external air-separation unit or internally in an integrated membrane reactor. One major disadvantage of using air-separation units for these kind of processes is that they consume a great amount of energy, thus penalizing the efficiency and power output of the plant. From the literature it can be found that the energy demand for air-separation units is as high as 0.3 kWh/kg O2 produced. The energy consumption for separating the oxygen from the air can be decreased very much if oxygen-separating membranes are used. Also this technique has a few disadvantages, namely: metal to ceramic sealing is needed that can withstand temperatures >800° C., the turbine inlet temperature (TIT) and the ceramic sealing temperature are linked, which limits the maximum TIT and thus lowers the performance of the plant and one needs to separate large amounts of air, corresponding to the total O2 required for full oxidation of fossil fuel powering the gas turbine.